Autoxidative and Photooxidative Reactivity of Highly Branched Isoprenoid (HBI) Alkenes

Abstract

Autoxidation of several mono-, di-, tri- and tetra-unsaturated highly branched isoprenoid (HBI) alkenes was induced in organic solvents using a radical initiator and enhancer, and their degradation rates were compared to those of classical phytoplanktonic lipids (mono-unsaturated fatty acids, sterols and chlorophyll phytyl side-chain). Autoxidation of two HBI trienes was also investigated in senescent and highly photodegraded diatom cells, collected in the Antarctic, using Fe(2+) ions as radical inducers. Autoxidation rates of HBI alkenes were found to increase with the number of tri-substituted double bonds, as expected. Further, HBI trienes possessing one bis-allylic position (where hydrogen abstraction is highly favoured) were found to be particularly reactive towards autoxidation and degraded at similar rates compared to polyunsaturated fatty acids in diatom cells. By comparison of the autoxidation products of the most reactive tri-unsaturated HBI with the corresponding photooxidation products, some specific tracers of these two types of abiotic degradation processes were identified. The lack of reactivity of the mono-unsaturated HBI IP25 and a structurally similar di-unsaturated HBI towards autoxidative degradation supports the good preservation of these biomarkers in marine sediments.

Electron ionization mass spectrometry fragmentation pathways of trimethylsilyl derivatives of isomeric allylic alcohols derived from HBI alkene oxidation

“…[5,13] These compounds were photoand autoxidized as described previously. [10,11] Oxidation of alkenes 1 and 2 afforded 9-hydroperoxy-2,6,10,14tetramethyl-7-(3-methylpent-1,4-dienyl)-pentadeca-7(20E),-10(18)-diene (4) and 9-hydroperoxy-2,6,10,14-tetramethyl-7- (Scheme 1). In contrast, oxidation of 3 yielded 9-hydroperoxy-2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-6(17),10(18)-diene (7) and 9-hydroperoxy-2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-6(17),10Z/E-dienes (8 and 9) (Scheme 1).…”

“…The synthesis of deuterated analogues was not possible in the case of oxidation products of HBI 1 and 2 due to the instability of hydroperoxides 4-6 (resulting from the presence of another highly reactive tri-substituted double bond). [10,11]…”

“…Phytoplankton cells were collected from Commonwealth Bay (East Antarctica) as described previously. [11] Concentrated phytoplankton cell suspensions were stored frozen (-20°C) prior to analysis.…”

“…Due to the presence of two highly photo-reactive tri-substituted double bonds in HBI 1 and 2 (Scheme 1), it was not possible to quantify compounds 10-12 directly using standards, since they are not accumulated during photo-sensitized oxidation of 1 and 2 as a result of rapid further oxidation. [11] We therefore employed photoproducts of the related HBI 3 (possessing only one reactive tri-substituted double bond) (Scheme 1) and a correction factor that took into account the proportion of the selected precursor ion in the EIMS spectra of each compound and that of the selected MRM transition in each CID product ion spectrum. The limit of detection (LOD) of the method, defined as the lowest concentration at which the signal-to-noise ratio is at least 5, was 0.01 μg g À1 .…”

“…and Berkeleya spp. [2][3][4][5][6][7][8][9] Mechanisms for the photo-and autoxidation of a number of HBI alkenes in senescent diatoms have been studied previously [10,11] and the rates of these processes shown to be strongly dependent on the number, the position and the geometry of the double bonds, with isomers containing tri-substituted double bonds and bis-allylic positions being especially reactive. These processes mainly produce isomeric epoxides and allylic hydroperoxides, which could fulfill the role of specific tracers of early diatom senescence; a primary process involved in the recycling of marine organic carbon.…”